1. Field of the Invention
One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a driving method thereof, or a fabrication method thereof. One embodiment of the present invention relates to an organometallic complex. In particular, one embodiment of the present invention relates to an organometallic complex that is capable of converting a triplet excited state into luminescence. In addition, one embodiment of the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each using the organometallic complex.
2. Description of the Related Art
In recent years, a light-emitting element using a light-emitting organic compound or inorganic compound as a light-emitting substance has been actively developed. In particular, a light-emitting element called an electroluminescence (EL) element has attracted attention as a next-generation flat panel display element because it has a simple structure in which a light-emitting layer containing a light-emitting substance is provided between electrodes, and characteristics such as feasibility of being thin, lightweight, and highly responsive to input signals, and able to be driven with direct current at low voltage. In addition, a display using such a light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Further, since such a light-emitting element is a plane light source, the light-emitting element is considered applicable to a light source such as a backlight of a liquid crystal display and lighting.
In the case where the light-emitting substance is an organic compound having a light-emitting property, the emission mechanism of the light-emitting element is of a carrier-injection type. That is, by applying voltage with a light-emitting layer provided between electrodes, electrons and holes injected from the electrodes recombine to put the light-emitting substance into an excited state, and light is emitted when the excited state returns to a ground state. There are two types of the excited states that are possible: a singlet excited state (S*) and a triplet excited state (T*). In addition, the statistical generation ratio of S* to T* in a light-emitting element is thought to be 1:3.
In general, the ground state of a light-emitting organic compound is a singlet state. Light emission from the singlet excited state (S*) is referred to as fluorescence where electron transition occurs between the same multiplicities. On the other hand, light emission from the triplet excited state (T*) is referred to as phosphorescence where electron transition occurs between different multiplicities. At room temperature, observations of a compound which emits fluorescence (hereinafter referred to as a fluorescent compound) usually show only fluorescence without phosphorescence. Therefore, the internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including the fluorescent compound is assumed to have a theoretical limit of 25%, on the basis of S*:T*=1:3.
On the other hand, when a phosphorescent compound is used as a light-emitting organic compound, the internal quantum efficiency can be theoretically increased to 100%. In other words, the emission efficiency can be four times as much as that of the fluorescent compound. For this reason, light-emitting elements using phosphorescent compounds have been recently under active development so that high-efficiency light-emitting elements can be achieved. As the phosphorescent compound, an organometallic complex that has iridium or the like as a central metal has particularly attracted attention because of its high phosphorescence quantum yield; for example, an organometallic complex that has iridium as a central metal is disclosed as a phosphorescent material in Patent Document 1.
An advantage of use of the highly-efficient light-emitting element is that power consumption of an electronic device using the light-emitting element can be reduced, for example. With recent attention to the energy problems, power consumption is becoming a major factor affecting the trends in consumer purchases and thus attains considerable importance.